Abrasive jet machining may be defined as the removal of material through the action of a focused, high velocity stream of fine grit or powder-loaded gas. Basic apparatus for carrying out the abrasive jet machining process may include an air or gas supply, a powder reservoir with cooperating vibrating means, a mixing chamber, and a conduit for feeding the resultant powder-gas mixture to a hand held piece having an appropriate nozzle for directing the powder-gas mixture against a workpiece for delicate removal of portions thereof. A foot switch may control the gas pressure at the nozzle. Under pulsation from the vibrating means, the powder trickles into a pressurized air stream, the amount of powder delivered being proportional to the amplitude of vibration, among other factors.
The abrasive powder, such as aluminum oxide and silicon carbide, for example, should be well classified and clean, and typically ranges in size from about 10 to 150 microns in diameter.
Abrasive jet machining is not a mass material removal process, but one of finishing. Because of the small amount of abrasive powder flowing through the nozzle at any given instant, it is not difficult to remove selected portions of the workpiece. Constant motion of the hand held piece and nozzle is recommended to lessen the risk of forming unwanted grooves or imperfections on the workpiece surfaces.
Current vibrators however have a tendency to produce unreliable powder flow rates resulting in an excess or deficiency of abrasive powder particles in the powder-gas mixture, and hence, a quality of finish on the workpiece which may not be satisfactory, whether the finishing is accomplished manually or by electronic control.
The present invention provides apparatus which supplies dry powder in controllable amounts from a powder supply chamber to a mixing chamber where the resultant air-powder mixture is fed into a line for transport to a site requiring the mixture. The apparatus may be advantageously employed in any application where it is desired or required that dry powder be supplied, conveyed, metered or transported in controllable amounts, for example, in abrasive jet machining processes aforedescribed, in dry powder coatings for electrostatic painting processes, and the like.
The present invention provides apparatus which meters dry powder from a pressurized powder supply chamber by means of a throttling device responsive to voltage. More specifically, the application of a direct current voltage to a pair of piezoelectric elements bonded to opposing faces of a thin cantilevered metallic beam, as in a bimorph bender, causes the beam to deflect upwardly or downwardly depending upon the polarity of the applied dc voltage to the polarized piezoelectric elements. A tapered plug is bonded to the free end of either the beam or upper piezoelectric element such that deflection of the beam causes the plug to traverse a passageway connecting the powder supply chamber with the mixing chamber to thereby vary the size of the passageway opening and hence the amount of powder capable of falling or flowing therethrough.
The plug is provided with a thin upstanding rod which penetrates the passageway to contact the powder in the powder supply chamber. Since the dry powder may not fall freely of its own accord through the controlled opening of the passageway, the dc bias voltage which controls the amount of deflection of the beam is impressed with an ac component, typically 60 H.sub.z, from a fixed frequency control oscillator, for example, which causes the deflected beam to vibrate, which, in turn, vibrates the plug and rod to help insure the free flow of the powder.